1. Field of the Invention
The present invention relates to a molding device for forming an ingot from a molten material, and a method of forming an ingot from a molten material.
2. Discussion of the Background
As a clean energy source alternative to petroleum oil, practical use of solar batteries has been sought in an extensive field ranging from a small-scaled house hold use to a large-scaled power generation system. These solar batteries are classified into a crystal type, an amorphous type, and a compound type in accordance with types of materials used, and among them, a majority of the solar batteries currently distributed in the market are crystal type silicon solar batteries. These crystal type silicon solar batteries are further categorized into a single crystal type and a polycrystalline type. In a single crystal silicon solar battery, since the substrate has a good quality, it is easy to make the conversion efficiency higher. However, a cost of manufacturing the substrate is high. On the other hand, the demand of the polycrystalline silicon solar battery has been increased while the concern for the environmental issues is growing, and the polycrystalline silicon solar battery has been desired to have higher conversion efficiency at much lower cost.
In the polycrystalline silicon solar battery, there are methods of forming polycrystalline silicon ingots as follows. In one method, a molten material obtained by heat-melting silicon is poured into a molding device, and the molten material is solidified in one direction from the bottom of the molding device, to thereby form the silicon ingot. In another method, a raw silicon material is supplied into a molding device and melted, and then the molten material is solidified from the bottom of the molding device, to thereby form a polycrystalline silicon ingot. In the ingots obtained as described above, their side portions or bottom portion contain many defects and impurities, and impurities due to solidification segregation are concentrated in the top portion of the ingot. Those portions are usually cut and removed in several mm or more in thickness, and by slicing those ingots thin using a multi-wire saw or the like, they are processed into polycrystalline silicon substrates for solar batteries.
In “A NEW DIRECTIONAL SOLIDIFICATION TECHNIQUE FOR POLYCRYSTALLINE SOLAR GRADE SILICON”, 15th Photovoltaic Specialist Conf. (1981), P576˜P580, there has been used a molding device which is made of graphite and silicon dioxide such as quartz and fused silica, and in which a mold release material coating containing a silicon nitride, a silicon dioxide, or the like is formed inside. In the molding device made of fused silica, during a process of cooling a block from the high-temperature state, the molding device is cracked due to the temperature gradient and difference of the sintering state inside the molding device, making it difficult to recycle the molding device. Also, in the molding device made of the quartz, a surface layer of the inner surface of the molding device coated with the mold release material becomes cristobalite, and the surface layer is peeled, causing wearing of the molding device and making it extremely difficult to recycle. Thus, the yield of the manufactured silicon ingots is poor.
Also, in Japanese Unexamined Patent Publication No. 10-190025, in order to remove a silicon ingot from an integral type molding device without breaking the molding device, there has been proposed a method for providing a larger taper on the inner surface of the molding device. Accordingly, the side surfaces of a silicon ingot cast by this molding device have an inverted taper expanding from the bottom of the ingot toward the top, thereby increasing edge portions which cannot be used as products. Therefore, expensive raw silicon materials are removed excessively.
In Japanese Unexamined Patent Publication No. 62-108515, there has been proposed a method for forming a molding device, in which high purity graphite is used for forming a bottom member and side members in plate forms, and then these bottom and side members are assembled and fixed by screws.
In Japanese Unexamined Patent Publication No. 6-144824, there has been proposed a method in which a silica powder layer is provided in a mold release material coated inside a molding device, and thus a stress due to the solidification expansion of silicon is absorbed by the softening deformation of the silica powder layer. Further, in Japanese Unexamined Patent Publication No. 10-182285, there has been proposed a method in which a wall thickness of a side member of a molding device is varied, utilizing that a surface with a small thickness is easily deformed and that a direction of a stress generated due to the solidification expansion of silicon is set to be in a direction of a tensile stress of a screw, to thereby prevent breakages of the screws.
The contents of the aforementioned references are incorporated herein by reference in their entirety.